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Thread: RTH6580WF with C Wire From 24v Adapter

  1. #1
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    RTH6580WF with C Wire From 24v Adapter


    I have a central AC that is currently connected to a battery powered Honeywell thermostat, it has a R, Y and G wire. R and Rc is connected with a jumper. AC works fine.

    I switched to the new WiFi thermostat RTH6580WF by connecting the respective wires to the new terminal. Removed the Jumper between R and Rc, then connected one of the adapter wire to C and the other to Rc, plug the adapter to an outlet, the AC does not come on. I tried changing menu 1 to 4 which is for cooling only but the AC still doesn’t come on.

    I put the wires back to the old thermostat and the AC worked fine.

    Please advice.


  2. #2
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    That trick only works properly for heat only applications as the Rc needs to connect to the air handler.

    I assume that you don't have a spare wire for C. Some repurpose the wire that is doing G for C. Then jump Y and G at the air handler. Otherwise you could use a Venstar Add-a-wire.

    A risky way is to double up connections for the transformer but usually works. Connect both the R from the air handler and one of the wires from the new transformer (adapter) all together to the R/Rc terminals.

  3. #3
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    I actually had accidentally tried this before posting, left the jumper connected between R/Rc, R wire connected to the R terminal, then put one of the transformer wire to the Rc terminal; the AC didn't come on. I only tried it once, how risky is it to try again?

    If it's safer to do the G to C substitution, I will do that instead; I did find a few videos on this after reading your reply. Thanks!

  4. #4
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    To use the transformer you need to attach both wires. One to the C terminal and one to the R/Rc along with the R from the HVAC system.

    G to C transition is safer as long as you hook it up right. And if you don't you should only blow a fuse. Adding an extra transformer is more risky mainly if done wrong. If you hook it up wrong you will send 48vac to the furnace control board and break it. One tech has stated that even when hooked up properly he is seeing more failures.

  5. #5
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    Click image for larger version. 

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    Hi, I went up to the attic where the air handler is located and took a picture of the wiring - first time i actually look at this, not sure I have any comment, lol.

    Let me to try explain the wiring but I am sure you will comprehend better than me - the brown cable on the left with 3 wires; red (R), green (Y) and yellow (G) come from the thermostat. the brown cable on the right with 2 wires; red and yellow come from the air handler. Then there are 2 blue wires coming from i believe the circuit breaker? Finally, there is a transformer with a C (top) and R (bottom) terminal.

    The thermostat wires with the Red (R) connected to the R terminal, Yellow (G) connected to the Red of the air handler, while the Green (Y) is connected to one of the blue cable from the circuit breaker (it's all the way in the back not sure if its clear to see). Then the yellow from the air handler as well as the other blue cable from the circuit breaker is connected to the C terminal.

    I hope I got the wiring correctly. Based on the G substitution for C, I would have to take the yellow (G) wire in the thermostat and connect to the C terminal in the thermostat. Then goto the air handler and connect the yellow (G) wire to the C terminal of the transformer, and finally connect the red wire from the air handler to the green (Y) from the thermostat and blue cable from the circuit breaker?

    I could just turn off the circuit breaker here?

    Last edited by thermonewbie; 08-21-2020 at 08:23 PM.

  6. #6
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    Yes, first turn off the circuit breaker before moving any connections. No guarantee but that breaker should cut power to the transformer so you don't short out anything. Yes, your approach to changing the wiring seems correct.

    That is a strangely wired old system. Normally the transformer and disconnect switch is in the air handler. Three electrical cables going to the breaker disconnect box is also strange - one the supply, one to the air handler, and assume one going to the outside compressor with the blue wires switching via the black relay on the bottom. Every system I have seen has had the outdoor compressor relay outside in the unit there so the 24vac wires need to go out to it.

    It doesn't matter but verify the wire colors at the thermostat as the convention is for Y to be yellow and G to be green.
    Last edited by Stuff; 08-22-2020 at 08:30 AM.

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